A fuel tank for a motor vehicle may typically comprise a tank body with a reservoir arranged therewithin. The reservoir is normally part of a fuel delivery unit for supplying the engine of the motor vehicle with fuel such as gasoline or diesel. In that respect, the reservoir which is of a volume that is substantially smaller than that of the fuel tank overall is intended in particular to ensure the function that, even under adverse operating conditions, for example when the motor vehicle in which the fuel tank is installed is negotiating a curve or bend of considerable length or when the vehicle with the fuel tank is in a position that is not horizontal, there is always an adequate amount of fuel in the region of the intake for the fuel pump. That presupposes the presence of a certain minimum amount of fuel in the reservoir and thus a corresponding minimum volume for the reservoir itself.
In the case of tanks which are produced in one piece, for example those which are made from thermoplastic material, preferably by means of extrusion blow molding, in many cases the necessity arises for the reservoir to be introduced into the tank, after manufacture thereof, through an opening which is subsequently formed in the wall of the tank body. As for example for reasons of stability and sealing integrity of the opening which is to be closed after the reservoir has been fitted into the tank, it is desirable for the opening to be kept as small as possible, previous tanks of this kind suffer from the disadvantage that the reservoir which is to be fitted into the tank interior through the opening is of a correspondingly small volume which for example can be of the order of magnitude of between 0.5 and 1 liter. In many cases, having regard to the duration of the above-mentioned adverse operating conditions which can occur in a practical context and during which only a small amount of or no fuel flows into the reservoir out of the tank body volume surrounding the reservoir, the above-indicated reservoir volume is too small. It is possible to counter that disadvantage by the reservoir being of a deformable nature, at least over portions of the wall defining same, in such a way that, after it has been inserted into the fuel tank body, it experiences a change in shape which results in an increase in its volume and a change in its cross-section within the tank body. A procedure of that kind can be applied in a practical situation however only when the reservoir with the associated components is designed in a manner which permits easy handling thereof both when fitting the reservoir in the tank and also in terms of the measures which result in the desired increase in the volume of the reservoir.